1. Field of Invention
The invention relates to a control apparatus and control method for an internal combustion engine, and more particularly to a control apparatus and control method for an internal combustion engine in which an octane value of a fuel being used is changed.
2. Description of Related Art
In a typical gasoline engine, knocking is more likely to occur as an octane value of a fuel becomes lower. When a low-octane fuel is used in the gasoline engine, ignition timing is usually retarded compared to the ignition timing at which optimal performance can be obtained if a standard-octane fuel is used. The ignition timing is retarded by an amount based on the octane value of the low-octane fuel so as to prevent occurrence of knocking.
However, as the ignition timing is retarded, an exhaust gas temperature increases in the gasoline engine. Therefore, when the ignition timing is retarded in order to prevent occurrence of knocking as described above, the temperature of components of an exhaust system may excessively increase due to an increase in the exhaust gas temperature during high-load operation or the like, which may lead to a decrease in the effective lifetime of the components of the exhaust system.
Thus, the temperature of the exhaust system is maintained at or below a predetermined temperature (for example, an upper limit temperature at or below which reliability of the components of the exhaust system can be maintained) by increasing an amount of the fuel injected to the engine such that an air-fuel ratio is reduced (i.e., the air-fuel ratio is made rich) in an engine operating region in which the exhaust gas temperature increases.
Japanese Patent Laid-Open Publication No. 2001-50081 discloses an example of an engine in which the exhaust system temperature is controlled by increasing the fuel injection amount as described above.
In the engine disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 2001-50081, since the fuel injection amount is increased based on the octane value of a fuel, in an engine operating region in which overheating of an exhaust system needs to be prevented, it is possible to increase the fuel injection amount by an appropriate amount even when the octane value of the fuel is changed. Thus, it is possible to prevent a decrease in fuel efficiency due to overheating of the exhaust system or an excessive increase in the fuel injection amount.
Meanwhile, in the engine disclosed in the Japanese Patent Laid-Open Publication No. 2001-50081, since the amount by which the fuel injection amount is increased is decided based on the octane value of the fuel, the fuel injection amount needs to be increased by a larger amount as the degree of the increase in the exhaust system temperature due to retard of the ignition timing becomes larger, when the octane value of the fuel becomes lower.
However, in the case where the fuel injection amount is increased by a larger amount in order to suppress the increase in the exhaust system temperature as the octane value of the fuel is lower, an air-fuel mixture in a combustion chamber may become excessively rich and a misfire limit may be reached. This is particularly possible in an operating state where the exhaust gas temperature becomes high and a low-octane fuel is used.
For example, a fuel supply device may supply at least two fuels having greatly different octane values, such as a high-octane gasoline having an extremely high octane value and a low-octane gasoline having an extremely low octane value, in a given proportion, and the proportion may be changed according to an engine operating condition or the like so that the octane value of the entire fuel supplied to the engine can be changed according to the engine operating condition or the like while the engine is operated. In this case, the octane value of the fuel being used varies within a large range.
Accordingly, in the case where the increase in the exhaust system temperature is suppressed by increasing the fuel injection amount during the high-load operation or the like and when the octane value is lowest, the fuel injection amount needs to be increased by a large amount, which may lead to occurrence of a misfire in a combustion chamber due to a rich air-fuel ratio.